In the past week, about 900 people have been killed -- mostly in Cairo -- after supporters of ousted President Mohamed Morsy clashed with the military-backed interim government. Both sides blame each other for starting the violence.

Many of the Morsy loyalists are supporters of the Muslim Brotherhood. The Islamist group condemned the attack on Egyptian soldiers.

"Our peaceful protests (are) stronger than any weapon, and we don't accept any violence," said Murad Mohamad Ali, media adviser to the Brotherhood's Freedom and Justice Party.

In May, seven Egyptian solders were kidnapped and held for six days in the Sinai Peninsula, a spokesman for Egypt's armed forces said.